According to a conventional method for preparing and hardening carbon steels, a ferrite and carbide mixture containing carbon in the desired percentage by weight is heated to the "austenitizing" temperature range. As used herein high carbon steel refers to steel having a carbon content of 0.6% by weight or greater. In the starting ferrite, iron atoms assume a cubic crystal structure with an additional iron atom at the center of the cube. The ferrite crystal structure is referred to as a body centered cubic structure and can dissolve and retain only 0.025% by weight carbon at temperatures up to 1341.degree. F. (727.degree. C.). In order to obtain high carbon steels of greater hardness and strength having a dissolved carbon content greater than, for example, 0.6% by weight carbon, the ferrite is heated to the austenitizing temperature range at which the crystal structure transforms from a body centered cubic form to a face centered cubic form having an additional iron atom at the center of each face of the cube. This new transitional crystal structure of iron or steel is referred to as austenite and is capable of dissolving up to a limit of, for example, 2.1% carbon at an austenitizing temperature of, for example, 2000.degree. F. (1093.degree. C.). The fully austenitizing temperature range for typical low alloy steel compositions is, for example, 1500.degree. F. (815.degree. C.) to 2000.degree. F. (1093.degree. C.).
After the desired carbon content has been diffused and dissolved in the austenite, the austenite is rapidly quenched or cooled to a low temperature such as room temperature or lower. During the final stages of the rapid quenching and beginning at the martensite start temperature, the austenite transforms to martensite in which the carbon is retained in solution in the crystal structure. The martensite is a body centered tetragonal structure in which the carbon is retained without precipitation. It may be viewed as ferrite supersaturated with carbon. For typical low alloy steel compositions the austenite begins to transform into martensite in the range of, for example, 280.degree. F. (138.degree. C.) to 310.degree. F. (155.degree. C.) and in some instances as high as 400.degree. F. (205.degree. C.). The transformation of austenite to martensite is dependent on both the rate of change of temperature and the temperature. Upon quenching to room temperature or with cold treatment to even lower temperatures, most of the austenite is transformed to martensite. Martensite exhibits severe distortion and stress of the crystal lattice as a result of the carbon incorporated by solution in the martensite structure.
According to the conventional method for preparing and hardening martensite steel, the martensite is then subject to a final tempering step in which the martensite is heated to a temperature in the range, for example, 400.degree. F. (210.degree. C.) to 1100.degree. F. (600.degree. C.). During the final tempering, the martensite decomposes into a carbide precipitate such as cementite in a ferrite matrix. The tempered martensite therefore comprises two phases of ferrite and carbide or cementite in a fully dispersed mixture. The decomposition of martensite into cementite and ferrite is dependent upon temperature and is also dependent upon the time at the tempering temperature.
The martensite in the "as quenched" state prior to tempering exhibits increasing hardness with increase in carbon content and accompanying lattice stress. The highly stressed martensite exhibits undesirable brittleness. Upon tempering, there is some loss of hardness but greater toughness of the steel as the highly stressed martensite relaxes to ferrite and carbide. The lower the tempering temperature, the finer the structure and finer the particles of cementite imbedded in the ferrite matrix. A coarser structure is exhibited by higher tempering temperatures.
A number of disadvantages are encountered in forming and hardening high carbon martensite steels having a carbon content of 0.6% by weight or greater according to the conventional method. As the carbon content of the martensite increases, the crystalline form changes. At carbon contents above 0.6% by weight the crystals form plate or disc microstructures which are prone to crack. Such cracks, which are confined to single crystals of martensite are defined and referred to herein as microcracks. Microcracking is believed to result from impingement of growing martensite crystals against one another during the martensite transformation. In the untempered, as quenched condition, the martensite transformation produces regions of high stress where plates impinge upon one another. These stresses are relieved by microcracking. Thus, some of the lattice stresses in the martensite are relieved by microcracking during quenching. While subsequent tempering relaxes or relieves the stress in the remaining martensite structure, microcracks formed during the martensite transformation remain in the tempered structures.
Thus, in the ideal method the desired carbon content is incorporated in the steel by completely diffusing and dissolving the carbon at the full austenitizing temperature followed by substantially complete transformation of the austenite to martensite in a rapid quench to room temperature or with cold treatment to lower temperature. The rapid quench or a marquench effectively retains the carbon in solution in the martensite structure viewed as ferrite supersaturated with carbon. The purpose of the martensite transformation prior to tempering is to provide greatest homogeneity of carbon distributed in iron or steel. The martensite further provides the greatest dispersion of carbon in the iron or steel structure upon tempering. Upon tempering, fine grains or particles of carbides such as cementite precipitate while the martensite relaxes to ferrite providing carbon steel with greatest homogeneity and mixture of the two phases. The technological difficulty that is encountered in applying the otherwise ideal conventional method of martensite hardening to high carbon content steel is of course the occurrence of microcracking. The solution to the problem of microcracking in high carbon martensite has proved intractable. Two approaches have developed for minimizing or reducing microcracking.
According to a first method, the amount of carbide dissolved in austenite to provide carbon in solution in the austenite structure is limited by controlling the austenitizing temperature and the time at austenitization. The austenite is then rapidly quenched without complete solution of the full carbon content of the steel. The austenite and transformed martensite contain undissolved or residual carbides so that the carbon content in solution in the martensite is relatively low, below, for example, 0.6% by weight carbon. As a result the low carbon content martensite formed during quenching and transformation of austenite to martensite is not subject to microcracking. In other words, high carbon content by weight is incorporated in the steel partly in the form of undissolved or residual carbides with a relatively low percentage of carbon dissolved in the martensite. However, the fracture toughness of steels with undissolved carbides in the microstructure is inferior. Fatigue resistance of high carbon steels with residual carbides in the martensitic microstructure is also lower. A description of this prior art method which seeks to achieve finer dispersion of undissolved carbide in the final structure is found, for example, in U.S. Pat. No. 3,575,737.
This conventional approach is further exemplified in the Grange U.S. Pat. Nos. 3,337,376 and 3,891,474 which also seek to achieve very finely dispersed undissolved carbide in a matrix of low carbon content martensite in the "as quenched" state. The final tempered steel achieved by Grange is therefore only a partial percentage of tempered martensite mixed with a separate phase of dispersed carbides. As a result, Grange cannot achieve a truly homogeneous dispersion of carbide in ferrite after final tempering. Only a portion of the steel with relatively low carbon content is austenitized and quenched to martensite while the remaining portion is composed of an undissolved and dispersed carbide phase.
Moreover, the Grange U.S. Pat. No. 3,891,474 deals only with the carburizing of low carbon steel having a content of less than 0.4% by weight carbon. While the Grange U.S. Pat. No. 3,337,376 describes methods for hardening steel which contain 0.8% or more of carbon, the steel always comprises an undissolved carbide phase and is never subjected to a complete martensite transformation.
In summary, by this first conventional approach to reduce microcracking, the steel ends up with low carbon content in the austenitic phase, for example, less than 0.6% by weight carbon. Ordinary engineering low alloy high carbon steel conventionally includes part of the carbon in a separate carbide or cementite phase to avoid microcracking. Only a portion of the high carbon content is actually incorporated by solution in the austenite and martensite to avoid the microcracking which occurs in the martensite crystals with high carbon content in solution.
According to a second conventional method to control and reduce microcracking, the austenite grains achieved during austenitization to dissolve the carbides are limited in size. Limitation of the austenite grain size in turn limits the size of the corresponding martensite grains after quenching and martensite transformation. In order to achieve rapid austenitization to limit the time and therefore crystal growth or grain size in the austenite temperature range, the steel must be specially prepared, for example, by preheat treatment to have a homogeneous bainite or pearlite microstructure so that solution of a finally dispersed carbide phase can occur rapidly at austenitizing temperature. Rapid heating to the critical austenite temperature range must be followed by rapid quenching as soon as the carbide phase is dissolved in the austenite since rapid austenite grain growth occurs in this temperature range. For example, it may be necessary to heat at rate of 300.degree. F. per second and to hold the critical temperature for no more than 20 seconds. Thus, elaborate temperature control is required and even with this control microcracking still is present in the "as quenched" untempered martensite. This is true even when the martensite is transformed from austenite grain sizes as fine as ASTM No. 9. Moreover, the carbide phase is seldom completely dissolved because of the time limits imposed leaving an undissolved carbide phase. This more elaborate and more costly process is not conductive to widespread commercial application. A prior reference exemplifying this method of rapid heating and short holding time for controlling the grain size of the austenite is found in U.S. Pat. No. 3,271,206 for "Short Time Heat Treating Process for Steels".
An additional problem related to the occurrence of microcracks is that microcracks generally cannot be healed by further heat treatment, tempering or anealing. With increased temperature over 1000.degree. F. (538.degree. C.) there is some healing of microcracks but a large portion of residual microcracking remains.
Thus, the steelmaker is limited to a low carbon austenite and martensite, for example, as described in U.S. Pat. Nos. 4,067,756, 3,920,490, and 3,891,474; to the use of a high alloy steel for increased toughness and strength, for example, as set forth in U.S. Pat. Nos. 3,575,737 and 3,619,302; or to maintaining a significant portion of the carbon in a separate dispersed but undissolved carbide phase, for example, as set forth in U.S. Pat. Nos. 3,337,376 and 3,271,206.
To applicant's knowledge, no one has yet been able to achieve an ordinary engineering low alloy steel in the form of a high carbon martensite with essentially complete austenitization and transformation to martensite without significant microcracking.